For testing the measuring function of a measuring device for gas measurements it is known to use a special reference gas with a known concentration of the gas to be measured by the measuring device. For example, if the measuring device is used to measure the concentration of nitrogen monoxide in exhaled breath, a special reference gas with a known concentration of nitrogen monoxide in nitrogen is used. That is, the bulk gas is nitrogen which contains a specified concentration of nitrogen monoxide. This special reference gas is stored in compressed form in a gas cylinder and is then fed to the measuring device, often via a pressure regulator and a gas fitting. The reading of the measuring device is then compared with the known concentration of the gas to be measured in the special reference gas. One type of pressure regulator comprises 2 pressure meters, a valve for manually adjusting the output pressure of the regulator and a stop valve at the output of the regulator.
This technique is expensive because of the cost of the special reference gas. In some cases it is also difficult to manufacture the special reference gas within the required specifications. This is for example the case when a special reference gas with a low concentration of the gas to be measured is needed. It is also time consuming to acquire the special reference gas and the handling of the gas cylinder is inconvenient because of the size and weight of the gas cylinder.
A measuring device for gas measurements can functionally be seen as comprising two parts. A first part (gas processing part) that processes the gas to be measured, including a gas sensor giving at least one electrical output value, and a second part (measurement value processing part) that processes and displays or outputs the at least one output value from the gas processing part.